Rotolined Articles

ABSTRACT

The present invention relates to the field of steel pipes and vessels rotolined with metallocene-produced polyethylene.

The present invention relates to the field of steel pipes and vesselsrotolined with metallocene-produced polyethylene.

The technique of rotolining allows for a seamless polymer lining to beapplied to the interior surfaces of metal structures.

Rotomoulding is used for the manufacture of simple to complex, hollowplastic products. It can be used to mould a variety of materials such aspolyethylene, polypropylene, polycarbonate polyamide, or polyvinylchloride (PVC). Linear low density polyethylene is preferably used asdisclosed for example in “Some new results on rotational molding ofmetallocene polyethylenes” by D. Annechini, E. Takacs and J.Vlachopoulos in ANTEC, vol. 1, 2001.

Polyethylene represents more than 80% of the polymers used in therotomoulding market. This is due to the outstanding resistance ofpolyethylene to thermal degradation during processing, to its easygrinding, good flowability, and low temperature impact properties.

A variation of rotomoulding has been used for applying uniform lining tothe internal surfaces of metal structures, wherein the metal structureacts as the mould. The polymers that are typically used in rotoliningmust have excellent adherence to metal. They have been selected fromcross-linked polyethylene, or fluoropolymers such as for examplepolyvinyl difluoride (PVDF), ethylene-chlorotrifluoroethylene (ECTFE) orethylene-tetrafluoroethylene (ETFE) or ethylene vinyl alcohol (EVOH) orpolyamide (PA).

This approach of combining plastic and metal offers the advantages ofplastics such as corrosion resistance and cleanability with those ofmetals such as structural and dimensional stability. Any type of metalcan be rotolined provided the selected plastic lining adheres to itssurface.

Because of adhesion problems, polyethylene has not been used so far inrotolining. As it is the most commonly used material in rotomoulding, asit has desirable properties and as it is very cheap, it would desirableto use it in rotolining.

It is an aim of the present invention to prepare rotolined articleswherein the lining is prepared from metallocene-prepared polyethylene.

It is another aim of the present invention to prepare rotolined articleswherein the lining is homogeneous.

It is also an aim of the present invention to prepare rotolined articleswherein the polyethylene lining has an excellent adherence to the metal.

Any one of those aims is at least partially solved by the presentinvention.

Accordingly, the present invention discloses rotolined articlescomprising:

a) an outer skin layer prepared from an iron containing alloy;

b) an inner skin layer prepared from a polyethylene-based resin,comprising from 40 to 99.5 wt % of metallocene-prepared homo- orco-polymer of ethylene or a mixture thereof, based on the weight oflayer b) composition and from 0.5 to 60 wt % of a functionalised orgrafted polyolefin (FPO) or of an ionomer or of a polyolefin (PO)grafted to a non-polyolefin (NPO) in the form of a block copolymer(PO)g-NPO), or of a mixture thereof:

c) optional additional layers prepared from polyamide (PA), ethylenevinyl alcohol (EVOH), polypropylene (PP) or polyethylene (PE), andpositioned either between layer a) and layer b) or on the side of layerb) opposite to layer a) or a combination of both.

Preferably, inner skin layer b) comprises homopolymers or copolymers ofethylene and grafted polyethylene or ionomer or combination thereof. Inthis description, copolymers are prepared from one monomer and one ormore comonomers.

In a preferred embodiment according to the present invention, thecomposition of inner skin layer b) comprises at least 70 wt %,preferably at least 80 wt %, based on the weight of skin layer b)composition and at most 99.5 wt %, preferably, at most 99 wt % andpreferably at most 98 wt % of metallocene-prepared polyethylene. Itcomprises at least 0.5 wt %, preferably 1 wt %, more preferably 2 wt %and at most 30 wt %, preferably at most 20 wt % of functionalised orgrafted polyethylene or ionomer or mixture thereof.

The metallocene-prepared polyethylene can advantageously be a mixture ofmetallocene-produced polyethylene resins having different density and/ormelt index in order to tailor the properties of the polyethylenecoating.

The functionalised or grafted polyolefins, if present are polyolefinsgrafted with a material that provides polarity and/or reactivity andthey therefore depend upon the nature of the adjacent layers. Preferablyin the present invention, the polyolefins are grafted with anhydride andpreferably, the polyolefin is polyethylene or polypropylene, morepreferably, it is polyethylene. Alternatively, an ionomer is used aloneor in combination with the grafted polyolefin, preferably incombination. Functionalised or grafted polyethylene provides excellentadhesion properties whereas ionomers enhance mechanical properties. In amore preferred embodiment according to the present invention, a mixtureof ionomer and grafted polyethylene is used in the composition of innerskin layer b).

An ionomer is a polyelectrolyte that comprises copolymers containingboth electrically neutral repeating units and a fraction of ionisedunits, usually representing at most 15 percent of the polymer. They linkin such a way that, even though they are stiff at room temperature, thebonds may be broken down thermally and the new linkages will cause thematerial to act as though it were a thermoplastic material. The ionicattractions that result strongly influence the polymer properties,especially its mechanical properties.

In an ionomer, the nonpolar chains are grouped together and the polarionic groups are attracted to each other. This allows thermoplasticionomers to act in ways similar to that of cross-linked polymers orblock copolymers, but in fact they are reversible cross-linkers. Whenheated, the ionic groups lose their attractions for each other and thechains become mobile. The chains motion increases with increasingtemperature and the groups can no longer stay in their clusters. Thisproduces a polymer that has the properties of an elastomer and theprocessability of a thermoplastic.

Suitable ionomers can for example be selected frompoly(ethylene-co-methacrylic acid). This polymer is a sodium or zincsalt of copolymers derived from ethylene and methacrylic acid.

The preferred polyethylene according to the present invention is a homo-or co-polymer of ethylene produced with a catalyst comprising ametallocene on a silica/aluminoxane support. More preferably, themetallocene component is ethylene-bis-tetrahydroindenyl zirconiumdichloride or dimethylsilylene-bis(2-methyl-4-phenyl-indenyl) zirconiumdichloride. The most preferred metallocene component isethylene-bis-tetrahydroindenyl zirconium dichloride.

The melt index of the polyethylene resin preferably used in the presentinvention is typically at least equal to 0.5 dg/min, preferably of atleast 1 dg/min. It is preferably at most equal to 25 dg/min, preferablyof at most 20 dg/min. The melt flow index MI2 is measured following themethod of standard test ASTM D 1283 at a temperature of 190° C. and aload of 2.16 kg.

The homo- and co-polymers of ethylene that can be used in the presentinvention preferably have a density of at least 0.920 g/cc, preferablyof at least 0.930 g/cc. It is of at most 0.965 g/cc, preferably of atmost 0.960 g/cc. The density is measured following the method ofstandard test ASTM D 1505 at 23° C.

The polyethylene of the present invention may also have a bi- ormultimodal molecular weight distribution, i.e. they may be a blend oftwo or more polyethylenes with different molecular weight distributions,which can be blended either physically or chemically, i.e. producedsequentially in two or more reactors.

The polydispersity D of the polyethylene suitable for the presentinvention is in the range 2 to 20, preferably 2 to 15, more preferablyless than or equal to 10, and most preferably less than or equal to 6,the latter range being typically associated with the preferredmetallocene-prepared polyethylene resins. The polydispersity index D isdefined as the ratio Mw/Mn of the weight average molecular weight Mwover the number average molecular weight Mn.

The resins of the present invention may also comprise other additivessuch as for example antioxidants, acid scavengers, antistatic additives,fillers, slip additives or anti-blocking additives, processing aid.

The metal used as external skin layer in the rotolined article ispreferably steel.

In rotolining, the metal article is used as mould. Said mould is filledwith the pelletised composition used as internal skin layer, placed inthe oven, and rotomoulded.

Additional layers prepared from different materials can be added inorder to provide specific properties.

Said additional layers may be advantageously prepared frompolypropylene, polyethylene, polyamide, polyvinylidene fluoride orfluoropolymers containing functional groups, ethylene/vinyl alcohol.

Polypropylene that can be used in the present invention typically has amelt flow index MI2 of from 1.5 to 100 g/10 min. The melt flow index MI2is measured following the method of standard test ASTM D 1238 at atemperature of 230° C. and under a load of 2.16 kg.

The polyamides that can be used in the present invention are theproducts of condensation:

of one or more amino acids such as aminocaproic acid, 7-aminoheptanoicacid, 11-aminoundecanoic acid and 12-aminododecanoic acid of one or morelactams such as caprolactam, oenantholactam and lauryllactam;

of one or more salts or mixtures of diamines such ashexamethylenediamine, dodecamethylenediamine, meta-xylylenediamine,bis(p-aminocyclohexyl)methane and trimethylhexamethylenediamine withdiacids such as isophthalic acid, terephthalic acid, adipic acid,azelaic acid, suberic acid, sebacic acid and dodecanedicarboxylic acid.

As examples of polyamides, mention may be made of PA 6 and PA 6-6.

It is also advantageously possible to use copolyamides. Mention may bemade of the copolyamides resulting from the condensation of at least twoα,ω-aminocarboxylic acids or of two lactams or of one lactam and oneα,ω-aminocarboxylic acid. Mention may also be made of the copolyamidesresulting from the condensation of at least one α,ω-aminocarboxylic acid(or a lactam), at least one diamine and at least one dicarboxylic acid.

As example of copolyamide, one can cite PA 6/12 and PA 6/6-6.

Mixtures of polyamide and polyolefins can also be used as additionallayer(s). The polyamide is as disclosed hereabove. The polyolefin can bea functionalised or a non-functionalised polyolefin or can be a mixtureof at least one functionalised and/or of at least one non-functionalisedpolyolefin.

More generally, amine terminated materials can also be used in theadditional layer(s) and they are preferably selected from polyamidediamine (PAdiNH₂).

The polyamides can also be impact-modified polyamides or foamedpolyamides.

Additional layer(s) may further advantageously be prepared frompolyurethane or from a composition comprising polyamide andethylene/vinyl alcohol copolymers (EVOH), or from ethylene/vinyl alcoholcopolymer alone and more generally, from a composition comprisingpolyamide and a barrier layer.

Such multiple layer linings can be prepared either by manualintroduction of material during the moulding cycle, or by the use of adrop-box, or by a one-shot system.

Manual addition involves moving the mould from the oven, removing a venttube or plug that creates an opening in the part and adding morematerial using a fennel or wand. This operation must be repeated foreach additional layer.

A drop-box typically contains a single material layer and it is aninsulated container that holds material until it is released at theappropriate time during the cycle. The signal for release of material isusually transmitted as a pressure pulse via the airline through the armof the machine. The insulation must be kept cool to prevent the materialinside the box from melting.

In either method, there are two critical factors:

the temperature at which the subsequent layer is added: it is criticalfor determining the wall thickness of the previous skin formed and howwell the two layers may be bound together;

the time elapsed before addition of the subsequent layer of material: ifthe mould is at rest for too long, material that has already adhered tothe wall may sag.

It is possible to reduce these problems by lowering the melt index ofthe first layer and/or by reducing the injection temperature of the nextlayer, and/or by cooling the mould slightly before injection or the nextlayer.

The cycle time necessary to produce rotolined articles depends upon themass injected for each layer.

The present invention also discloses a method for preparing rotolinedarticles that comprises the steps of:

a) providing the metallic hollow article to be lined as a mould;

b) feeding the the composition of the inner skin layer into the mould;

c) placing the filled mould in pre-heated oven;

d) rotating it about two perpendicular axes;

e) optionally repeating the operation with the desired additional layerseither before and/or after inner skin layer b);

f) cooling the mould;

g) retrieving the lined metal article.

Typically, the oven temperature is of from 280 to 300° C. Thetemperature inside the mould is preferably of the order of 160° C.

The mould is typically cooled with cold air.

The present invention is particularly suitable for preparing rotolinedsteel pipes or vessels.

1-8. (canceled)
 9. A rotolined article comprising: an outer skin layerprepared from an iron containing alloy; an inner skin layer preparedfrom a polyethylene-based resin, consisting of a metallocene-producedhomopolymer, copolymer of ethylene, or mixtures thereof, in amountsranging from 40 to 99.5 wt % based on the total weight of the inner skinlayer; and a material selected from the group consisting of afunctionalized polyolefin (FPO), an ionomer, and a polyolefin (PO)grafted to a non-polyolefin (NPO) in the form of a block copolymer(PO)g-(NPO), and combinations thereof, in amounts ranging from 0.5 to 60wt % based on the total weight of the inner skin layer.
 11. Therotolined article of claim 9, further comprising at least one additionallayer prepared from materials selected from the group consisting ofpolyamide (PA), ethylene vinyl alcohol (EVOH), polypropylene (PP), andpolyethylene (PE), and combinations thereof, wherein the at least oneadditional layer is positioned between the outer skin and the innerskin, on the side of the inner skin opposite to the outer skin, orcombinations thereof.
 12. The rotolined article of claim 9, wherein theiron containing alloy is steel.
 13. The rotolined article of claim 9,wherein the metallocene-produced polyethylene is prepared with abistetrahydroindenyl or a bisindenyl catalyst component.
 14. Therotolined article of claim 9, wherein the functionalized polyolefin isfunctionalized polyethylene.
 15. The rotolined article of claim 9,wherein the inner skin layer composition comprises polyethylene inamounts ranging from 80 to 98 wt % and from 2 to 20 wt % of afunctionalized polyethylene, an ionomer, or mixtures thereof.
 16. Therotolined article of claim 9, wherein the inner skin layer compositioncomprises a combination of functionalized polyethylene and ionomer. 17.The rotolined article of claim 9, wherein the inner skin layer has athickness ranging from 1 to 3 mm.
 18. A method for preparing therotolined article of claim 9, comprising: providing a metallic hollowarticle to be lined as a mold; feeding the inner skin layer compositioninto the mold to obtain a filled mold; placing the filled mold into apre-heated oven; rotating the filled mold about two perpendicular axes;cooling the mold; and retrieving the lined metal article.
 19. The methodof claim 18, further comprising repeating the method with a compositionselected from the group consisting of polyamide (PA), ethylene vinylalcohol (EVOH), polypropylene (PP), and polyethylene (PE), andcombinations thereof in place of the inner skin layer composition.